What do you think of [name_f]Leah[/name_f] and [name_f]Cora[/name_f] for a (hypothetical) sibset?
And also, would it seem odd to you to add a longer name to the set (i.e. [name_f]Leah[/name_f], [name_f]Cora[/name_f] & [name_f]Eleanor[/name_f] or [name_f]Leah[/name_f], [name_f]Cora[/name_f] & [name_f]Verity[/name_f]) or does it seem totally fine?
[name_f]Leah[/name_f], [name_f]Cora[/name_f], and [name_f]Antoinette[/name_f]
I think [name_f]Leah[/name_f] and [name_f]Cora[/name_f] are cute together. I think for a third sister name, it could be okay if the name is longer if it’s a name like [name_f]Verity[/name_f] that is three syllables but is only two letters longer. [name_f]Verity[/name_f] still looks pretty sleek and compact even though it is a syllable longer than [name_f]Leah[/name_f] and [name_f]Cora[/name_f], and I think it goes well. [name_f]Eleanor[/name_f] feels heavier/has more impact than [name_f]Leah[/name_f] and [name_f]Cora[/name_f], and also shares a lot of the same sounds and letters.
[name_f]Faith[/name_f]
[name_f]Faye[/name_f]
[name_f]May[/name_f]
[name_f]Mavis[/name_f]
[name_f]Amorie[/name_f]
I think [name_f]Leah[/name_f] and [name_f]Cora[/name_f] are nice together. [name_f]Cora[/name_f] feels a little more vintage-chic (even though [name_f]Leah[/name_f] is an old name), but they have a similar tone.
To answer your second question, names don’t have to be the same length to match in tone or style. [name_f]Eleanor[/name_f] seems too similar to [name_f]Leah[/name_f], but I like [name_f]Leah[/name_f], [name_f]Cora[/name_f] and [name_f]Verity[/name_f]—[name_f]Verity[/name_f] marries the vintage-chic feel of [name_f]Cora[/name_f] with the pious/biblical connotations of [name_f]Leah[/name_f]. [name_f]Susanna/name_f and [name_u]Marian[/name_u] also come to mind.
[name_f]Leah[/name_f], [name_f]Cora[/name_f] and [name_f]Ivy[/name_f]
[name_f]Leah[/name_f], [name_f]Cora[/name_f] and [name_f]Olive[/name_f]
[name_f]Leah[/name_f], [name_f]Cora[/name_f] and [name_f]Hope[/name_f]
[name_f]Leah[/name_f], [name_f]Cora[/name_f] and [name_f]Freya[/name_f]
[name_f]Leah[/name_f], [name_f]Cora[/name_f] and [name_f]Joy[/name_f]
[name_f]Leah[/name_f], [name_f]Cora[/name_f] and [name_f]Eve[/name_f]
[name_f]Leah[/name_f], [name_f]Cora[/name_f] and [name_f]Anna[/name_f]
[name_f]Leah[/name_f], [name_f]Cora[/name_f] and [name_f]Belle[/name_f]
I think it is perfectly fine if you put a longer name with two shorter names. As one PP mentioned I find [name_f]Leah[/name_f] and [name_f]Eleanor[/name_f] a bit similar (the way they look), but I think [name_f]Leah[/name_f], [name_f]Cora[/name_f] and [name_f]Verity[/name_f] make a lovely, girly sibset!
If it was three short names, like [name_f]Leah[/name_f], [name_f]Cora[/name_f], [name_f]Etta[/name_f] and then you wanted to do [name_f]Verity[/name_f], I’d be a little more uncertain. But with the sibset you have here I think its perfectly fine 
All the best!